Lubricants are often used in demanding environments where degradation of the lubricant base stock can lead to catastrophic results; and where such degradation is accelerated by high temperatures, extreme wear conditions, acidic or other corrosive conditions, etc. For example, automobile engines require periodic oil changes to replace degraded engine lubricant in order to protect against undue wear and engine damage.
Alkylated diphenylamines (ADPAs) are antioxidants widely used to prevent degradation and maintain the performance of modern engine oils found in gasoline and diesel engines for cars and trucks. Motor oil drain intervals have been significantly extended in recent years through the use of modern antioxidant formulations, coupled with improved base-stocks. Extending drain intervals conserves oil, reduces used oil and filter waste, and helps reduce the illegal disposal of used oil.
When selecting an ADPA antioxidant component for use a number of performance, safety and environmental concerns must be addressed. For example, diphenylamine itself has good antioxidant activity but it has long been known to be a sensitizer and its presence is typically kept to a minimum, e.g., less than 1% of any ADPA antioxidant. Further, diphenylamines substituted with essentially hydrocarbyl groups are more soluble in lubricating oil and the higher molecular weight reduces volatility. Increasing alkylation also helps to solubilize polar materials that form by oligomerization of spent oxidized amines reducing deposits, sludge and varnish.
On the other hand, the antioxidant activity of ADPAs is dependent on the concentration of nitrogen provided and is thus inversely proportional to molecular weight and excessive alkylation or very large alkyl groups should be avoided.
For an ADPA with a molecular weight of up to about 400 to 600 Daltons, other factors can be as important as nitrogen content. For example, alkylation of the aromatic ring at the para-position relative to the nitrogen is preferred as ortho-alkylation diminishes the activity of the amino group. Orthoalkylation, and ortho, para-alkylation should be minimized as much as possible. Since tri-alkylation of DPA overwhelmingly requires that one ring is ortho, parasubstituted, tri-alkylation is by definition undesirable.
When formulating a lubricant, liquid components, i.e., liquid at room temperature, e.g., 25° C., are typically preferred, as they are easier than solids to handle in the blending process. Liquids are also less likely to cause pour-point, gelling, or filter-clogging problems by crystallizing out of an oil than an additive that is itself a crystalline solid. Friedel-Crafts alkylation of diphenylamine is an equilibrium driven process and commercial ADPAs are often mixtures containing varying proportions of mono-, di-, and tri-alkylated diphenylamines, and tend to be liquid provided that certain ratio of the various products are obtained.
Also, as disclosed in U.S. Pat. No. 6,204,412, symmetrically disubstituted diphenylamines typically increase the melting point of the alkylated diphenylamine composition and can, in sufficient amounts, lead to a solid alkylated diphenylamine composition rather than a liquid. For example, a composition with 25 wt % or more dioctyldiphenylamine, obtained e.g., by reaction between diphenylamine and diisobutylene, is typically solid at room temperature. Thus, a balance must be found between the components of an ADPA mixture to ensure the proper product form.
There are processes in the art designed to provide an alkylated diphenylamine composition that is liquid at room temperature and contains a minimum amount of non-alkylated diphenylamine. For example, U.S. Pat. No. 6,204,412 discloses a method of manufacturing an alkylated diphenylamine composition comprising no more than 3 wt % unsubstituted diphenylamine, comprising steps of (a) reacting unsubstituted diphenylamine and at least one olefin comprising diisobutylene or other select linear alpha-olefins in the presence of a clay catalyst to form a reactive composition; and (b) adding a second olefin composition to the reactive composition to react with the reactive composition and generate an alkylated diphenylamine composition comprising no more than 3 wt % unsubstituted diphenylamine, wherein at least a portion of the second olefin composition is added to the reactive composition prior to forming 50 wt % mono-alkylated diphenylamine in the reaction mixture. The second olefin composition generally comprises isobutylene, styrene, or alpha-methylstyrene, which are more reactive with unsubstituted diphenylamine than the olefin of step (a).
U.S. Pat. No. 6,355,839 discloses a single step process for preparation of an alkylated diphenylamine antioxidant which comprises alkylating diphenylamine with polyisobutylene in the presence of a clay catalyst, wherein the polyisobutylene has an average molecular weight in the range of 120 to 600 and wherein the polyisobutylene contains at least 25% methylvinylidene isomer.
U.S. Pat. No. 6,315,925 provides a mixture of nonylated diphenylamines consisting essentially of, as measured by gas chromatography, a) from 68% to 78% by area dinonyldiphenylamine; b) from 20 to 30% by area nonyldiphenylamine; c) from 1.0 to 3.5% by area trinonyldiphenylamine; and d) from 0.1 to 1.0% by area diphenylamine; and a process for the preparation thereof by using acid catalysts in small quantities.
Methods are known that provide liquid ADPA mixtures with low non-alkylated diphenylamine content. Typically these mixtures comprise a significant amount, i.e., 20 wt % or more of mono-alkylated diphenylamine. However, approaches useful in reducing the amount of non-alkylated DPA in these mono-alkylated DPA rich diphenylamine compositions can be surprisingly unsuccessful and/or commercially unfeasible when applied to the reduction of mono-alkylated DPA concentrations in preparing liquid di-alkylated DPA compositions that are very low in mono-alkylated DPA. For example, attempts to convert mono-nonylated para-diphenylamine to a di-alkylated diphenylamine by reaction with diisobutylene resulted in the loss of nonyl groups, which can be replaced by alkylation with octyl groups forming the less desired t-octyl-diphenylamine. Reactions of mono-alkylated DPA with styrene or alpha-methylstyrene are potentially problematic due to environmental concerns.
The present invention provides liquid compositions with high amounts of di-alkylated DPA and low amounts of mono-alkylated DPA and a process for conveniently preparing said compositions. It is believed that the antioxidant compositions provided by the invention will have advantages in handling, compatibility and performance over other alkylated DPA compositions in many commercial applications.